Efforts to create smaller and lighter devices have recently intensified in the field of electric equipment, creating an urgent need for developing smaller and thinner cells for use with this equipment. For example, coin cells, button cells, and other cells are being widely used.
Cases for such cells are required to have adequate corrosion resistance, electrical conductivity, and deep drawability. Examples of materials used for anode cases include so-called three-layer clad materials in which stainless steel is used as the substrate, Ni is integrated with one principal plane of this substrate, and Cu is integrated with the other principal plane. In addition, so-called two- or three-layer clad materials in which stainless steel is used as the substrate, and Ni is integrated with one or both principal planes of this substrate are used for cathode cases.
With such button cells, battery life is determined by the amount of electrochemical reagent stored in the case, requiring that the capacity of the case be increased in order to extend battery life.
Some applications, however, impose restrictions on the outside dimensions of cells, forcing researchers to increase the actual case capacity by using thinner cases. It was impossible, however, to obtain case materials that would satisfy these requirements in terms of maintaining the mechanical strength of cases per se, preventing electrochemical reagents from leaking, or the like, making the aforementioned goal unattainable.
As a means of overcoming such shortcomings, it has been proposed to create button cells in which, in particular, the case capacity for accommodating electrochemical reagents is substantially increased by using as an anode case a three-layer clad material in which stainless steel is used as the substrate, Ni is integrated with one principal plane of the substrate, and Cu is integrated with the other principal plane, and setting the weight ratio of the stainless steel in the three-layer clad material to between 77% and 91% of the total amount of clad material (corresponds to a thickness ratio of 79% to 92%); and by using as a cathode case a three-layer clad material in which stainless steel is used as the substrate, Ni is integrated with both principal plane of the substrate, and the temper number of this three-layer clad material is set to a prescribed level (3.5) (Japanese Unexamined Patent Application (Kokai) 8-315869; U.S. Pat. Nos. 5,567,538, 5,582,930, and 5,591,541).
In such button cells, strength per unit of thickness of the clad material can be increased and the thickness reduced while the molding properties needed to form an anode case by pressing or the like can be maintained by increasing the ratio of the stainless steel constituting the substrate of the three-layer clad material that forms the anode case; and the capacity of the anode case can be increased while the strength, stiffness, and crushing resistance required for an anode case are maintained.
Although button cells constructed using anode cases and cathode cases composed of the above-described three-layer clad materials have much longer cell lives than conventional button cells, a need exists for a further increase in battery life and a creation of smaller and lighter cells, and further improvements in three-layer clad materials are desired. At present, however, it is difficult to achieve further increases in the weight ratio (thickness ratio) of the stainless steel constituting the aforementioned substrate.
A structure in which the Ni, Cu, or the like integrated with the principal planes of the stainless steel constituting the substrate is formed by plating is disclosed for the proposed anode case and cathode case described above.
In commercial-scale production, however, cold welding is commonly used to achieve fabrication because of considerations related to productivity, the costs incurred in handling plating equipment or plating solutions, and the like. Specifically, thin Ni or Cu sheets of prescribed thickness are superposed on the principal planes of stainless steel (substrate), and the sheets are pressure-welded and integrated at the same time with the aid of pressure rolls, rolling a three-layer clad material of prescribed thickness.
The creasing or the like of the thin Ni or Cu sheets during pressure welding is prevented by applying a specific amount of tension in the direction upstream of the pressure rolls with the aid of the rolls or other components for feeding these thin sheets, but these thin sheets commonly have a width of about 100 mm to 600 mm and a thickness of about 0.1 mm to 1.0 mm when produced on a commercial scale, so it is difficult to apply uniform tension across the entire thin sheet at a lower sheet thickness.
Consequently, the thin Ni and Cu sheets produce folds and creases when welded under pressure to stainless steel (substrate), ultimately creating surface defects and resulting in inadequate bonding with the substrate. It is therefore impossible to reduce the thickness of a thin Ni or Cu sheet below a certain limit, and, as a result, it is difficult to increase the weight ratio (thickness ratio) of stainless steel in the entire clad material above a certain level.
With an anode case, for example, the total thickness of the clad material is commonly required to be 0.30 mm or less. Conventional methods are therefore capable of ensuring that the weight ratio of Ni in relation to the total amount of clad material is 2% (thickness ratio: 2%), but are less successful in ensuring, in particular, that the weight ratio of Cu in relation to the total amount of clad material is less than 7% (less than 6% in terms of thickness ratio) and, ultimately, that the weight ratio of the combined amount of Ni and Cu in relation to the total amount of clad material is less than 9% (less than 8% in terms of thickness ratio).
It was thus assumed that the weight ratio (thickness ratio) of stainless steel (substrate) had insurmountable limitations and that it was difficult to increase battery life beyond that of a button cell having the above-described proposed structure, that is, a button cell in which the weight ratio of stainless steel was 77% to 91% (corresponds to a thickness ratio of 79% to 92%) of the total amount of clad material.